feat: update column definitions for owner and last_updated in migrations and models

Author: sokoliva

src/a2a/migrations/versions/6419d2d130f6_add_columns_owner_last_updated.py

@@ -35,6 +35,7 @@ def _get_inspector() -> sa.engine.reflection.Inspector:
 def _add_column(
     table: str,
     column_name: str,
+    nullable: bool,
     type_: sa.types.TypeEngine,
     value: str | None = None,
 ) -> None:
@@ -44,7 +45,7 @@ def _add_column(
             sa.Column(
                 column_name,
                 type_,
-                nullable=False,
+                nullable=nullable,
                 server_default=value,
             ),
         )
@@ -112,8 +113,8 @@ def upgrade() -> None:
     )
 
     if _table_exists(tasks_table):
-        _add_column(tasks_table, 'owner', sa.String(128), owner)
-        _add_column(tasks_table, 'last_updated', sa.DateTime(timezone=True))
+        _add_column(tasks_table, 'owner', False, sa.String(128), owner)
+        _add_column(tasks_table, 'last_updated', True, sa.DateTime())
         _add_index(
             tasks_table,
             f'idx_{tasks_table}_owner_last_updated',
@@ -126,7 +127,11 @@ def upgrade() -> None:
 
     if _table_exists(push_notification_configs_table):
         _add_column(
-            push_notification_configs_table, 'owner', sa.String(128), owner
+            push_notification_configs_table,
+            'owner',
+            False,
+            sa.String(128),
+            owner,
         )
     else:
         logging.warning(

src/a2a/server/models.py

@@ -151,7 +151,7 @@ class TaskMixin:
     )
     owner: Mapped[str] = mapped_column(String(128), nullable=False)
     last_updated: Mapped[datetime | None] = mapped_column(
-        DateTime(timezone=True), nullable=True
+        DateTime, nullable=True
     )
 
     # Properly typed Pydantic fields with automatic serialization

src/a2a/server/tasks/database_task_store.py

@@ -1,5 +1,6 @@
 import logging
 
+from datetime import datetime, timezone
 from typing import Any, cast
 
 
@@ -234,9 +235,13 @@ async def list(
             count_stmt = select(func.count()).select_from(base_stmt.alias())
             total_count = (await session.execute(count_stmt)).scalar_one()
 
-            # Use nulls_last() to ensure NULL timestamps sort last in descending order
+            # Use coalesce to treat NULL timestamps as datetime.min,
+            # which sort last in descending order
             stmt = base_stmt.order_by(
-                timestamp_col.desc().nulls_last(),
+                func.coalesce(
+                    timestamp_col,
+                    datetime.min.replace(tzinfo=timezone.utc),
+                ).desc(),
                 self.task_model.id.desc(),
             )